In an interview, Dr. Kapraun said he had administered 20,000 doses of the substance to horses over the years and continued to do so at off-track facilities, arguing that it heals tendon injuries quickly and safely. He acknowledged that a horse might benefit more from months away from competing. “The economics of horse racing does not allow for that. Horse racing is on the decline. If a horse needed a year to heal up, they would go to the killers up in Canada or Mexico,” he said, referring to slaughterhouse................

.........“There comes a time in every horse’s career that you’ve got to stand up and say, ‘That’s enough. We are endangering this horse,’ ” Dr. Ferraro said. “There are not a lot of veterinarians willing to do that.”

.......

It did not take long for word to circulate that a cheap, easily obtainable drug might make horses run faster if administered close to race time. Better yet, regulators might not suspect it.

So on April Fools’ Day in 2011, trainers at harness tracks tried it on nine racehorses. All finished in the money — eight in first place, one in second.

Over 12 days, with nearly $600,000 at stake at harness tracks in New York and Pennsylvania, 36 of 38 horses using the drug finished first or second, with 31 winning their races.

Another surprise was the drug itself, oxymetazoline, an ingredient in Afrin, an over-the counter cold medicine. Oxymetazoline, which is not approved for racing, stimulates a horse’s cardiovascular system when administered in large doses through an inhalation mask.

Some of the doped horses shared more than prize money. They also shared certain veterinarians, including Dr. Louis A. Grasso, according to New York racing commission records.

In addition to his felony conviction for selling steroids to weight-lifters, Dr. Grasso lost his New York State racing license for giving drugs too close to race time and signing blank scratch forms.

In 2000, Delaware authorities suspected him of treating horses without a license in the state, but when they tried to arrest him, Dr. Grasso led the police on a chase through the back roads of New Castle County. When they finally caught him, officers found needles, syringes and two banned drugs in his vehicle. He eventually pleaded guilty to resisting arrest.

Though New York’s Racing and Wagering Board has stripped Dr. Grasso of his license to practice at racetracks, it has no authority over his activities elsewhere.

The racing board also referred his conduct to the state’s veterinary board, which could bar him from practicing anywhere in the state. But it has not done so, leaving Dr. Grasso free to work at off-site training centers.